Question: If $a + 9b + 7c = -4$ and $5x + 4y + z = -6$, what is $-8y + 63c + 9a + 81b - 2z - 10x$ ?
$= 9a + 81b + 63c - 10x - 8y - 2z$ $= (9) \cdot (a + 9b + 7c) + (-2) \cdot (5x + 4y + z)$ $= (9) \cdot (-4) + (-2) \cdot (-6)$ $= -36 + 12$ $= -24$